1. Field of the Invention
This invention relates to a signal identifying device, a code book or codec changing device, a signal identifying method, and a code book or codec changing method, and more particularly, is applicable to a coding apparatus which can identify input signal and change code book used for coding or codec.
2. Description of the Related Art
Conventionally, techniques for compressive coding input signal such as voice signal at low bit rate have been proposed. A typical technique of signal coding at low bit rate is vector quantization. The most important characteristic of this vector quantization is in a point that while the conventional coding methods processes input signal as scalar amount, this vector quantization processes input signal as vector amount.
The vector quantization will be explained more concretely here. In the conventionally proposed coding methods such as Multiband excitation (MBE) coding, Singleband excitation (SBE) coding, Harmonic coding, Sub-band coding (SBC), Linear Predictive coding (LPC), or Discrete cosine transform (DCT), Modified DCT (MDCT), spectrum amplitude and other parameters obtained from input signal are used as information data and are processed as a scalar-amount to be quantized.
On the contrary, in the vector quantization, various information data obtained from input signal are not quantized as a scalar-amount individually, but a vector is respectively formed from a combination of several information data and information representing the vector (e.g., vector number) is coded. Accordingly, the vector quantization has the effects that bit rate can be remarkably lowered and quantization efficiency can be improved significantly, comparing to the case of the scalar quantization.
To practically realize the vector quantization, a plurality of typical vectors to which vector numbers are put are previously stored in a storing circuit such as a memory (hereinafter, the storing circuit in which the typical vectors are stored is referred to as code book.) which is prepared in a coding apparatus. In the coding apparatus, a vector is formed from a combination of several information data obtained from input signal, and the typical vector most similar to this vector is retrieved from the code book, and the vector number of the most similar typical vector is read to be coded. Thereby, only if the code book is prepared previously, the vector quantization can be realized easily.
In addition, in a decoding apparatus, if the same code book as the code book prepared in the coding apparatus is prepared, the corresponding typical vector is read from the code book based on the sent coded data (i.e., data of which vector number is coded), so as to easily perform decoding.
On the other hand, the input signal to be coded generally has the different characteristics depending on the signal type. When the vector quantization is performed, it is desired that the typical vector prepared as code book is a vector suitable for the characteristics of the input signal, also in order to reduce distortion generated by quantization. In other words, if the typical vector suitable for the characteristics of the input signal is prepared as code book, the coding characteristically suitable for the input signal can be performed. For instance, if the typical vector suitable for voice signal is prepared in the code book, the coding characteristically suitable for the voice signal can be realized, and if the typical vector suitable for music signal is prepared in the code book, the coding characteristically suitable for the music signal can be realized.
In connection, the voice signal described here is such signal that a main signal component is formed by "voice produced by the vibration of the human's vocal cords". The music signal is such signal that a main signal component is formed by "sound produced from one or more musical instruments".
Thereby, the code book suitable for this voice signal and the code book suitable for this music signal are prepared in the coding apparatus, and a user changes the code book or codec in accordance with the type of input signal, so as to perform coding with high grade suitable for the characteristics of input signal.
In the conventional coding apparatus, the code books suitable for the voice signal and the music signal are prepared so as to perform coding suitable for the characteristics of input signal. However, the apparatus is so designed that a user identifies input signal and changes the code book or codec. So, there is a problem that the user has to identify input signal and has to change the code book or codec. In other words, if input signal is automatically identified, usage of the apparatus will be improved significantly for users.